The present invention relates to an improved device for the DCT encoding of digital video signals.
As it is known, the conventional color digital video signal comprises a luminance component Y and two chrominance components Cr and Cb. The luminance component is sampled at 13.5 MHz, while each of the chrominance components is sampled at 6.75 MHz (CCIR. Recommendation 601, Dubrovnik 1986). Each sample of the single components is made of 8 bits. Thus the required bit per second flow rate for transmitting such a signal is 216 Mbit/sec i.e. 8*(13.5+6.75+6.75) eand, even with an elimination of the line and frame synchronisms transmission, the bit timing reduction cannot be over 25%. In the case of high definition video signals, the bit timing will be even 4 or 5 times the described one. It is thus apparent how the tramission of those signals would require high capacity transmission channels.
In order to reduce the bit rate of those digital video signals without significantly reducing the image quality, it is known to use the Discrete Cosine Transform (DCT) for encoding the video signal reducing the redundancy. In such encoding (as it will be more apparent from the following description) data are generated that are at least partially freed from the information tied to the spatial correlation of the image or also freed from the information connected to the time correlation of the image with respect to former images (frame or field) of the video signal. Consequently, the generated data flow is not uniform, as it can be very low in case of static images or very high in case of rapidly variable images. In these encoding systems, an output buffer is therefore provided, which is fed with a variable data flow and is emptied into the transmission channel with constant speed, depending on the transmission channel capacity.
In these encoding systems, the buffer is necessarily liable to occasional saturation, occuring the more often is the lower transmission channel capacity. This must be avoided because it may cause data to be lost with consequent decoding errors; for this purpose it is known to have a rough quantization of the DCT coefficients as the buffer is filled up. This naturally causes an increase of quantization noise and a deterioration of the reconstructed image. This effect is more evident when channels of reduced capacity are used, corresponding to an encoding of 1 to 1.5 bit/pel, for example, channels of 10-15 Mbit/sec, for noraml definition video signals (CCIR Recommendation N.601), or of 50-70 Mbit/sec for high definition video signals.